Excerpt
Alternation of generations is a dramatic and familiar evolutionary trend in the plant kingdom. It is a notable feature of land plants, important in studies of phylogenies and evolution of the diploid sporophyte. Alternation can be defined simply as the two phases that normally occur in the life cycle of a plant, one haploid (gametophyte) the other diploid (sporophyte).
Knowledge of plant life cycles and the study of land plant origins have grown significantly over the past century. Likewise, ideas on the origin of alternation have flourished. However, the traditional theories devised to explain alternation have languished, first in dispute, later in disregard because of confusing terminology and definitions that have not kept pace with changes in knowledge as it grew.
The homologous and antithetic theories were first described as different morphological patterns of reproduction homologous alternation meaning a cycle of similar forms in algae, vs. antithetic alternation, that seen in the regular pattern of differing phases in bryophytes and ferns. The theories soon came to mean opposing methods by which the sporophyte originated, thus resulting in alternation of generations.
Prior to 1900 the homologous theory derived the sporophyte by a change in reproductive mode of the gametophyte, a transfer of function, from production of gametes to producing spores for dissemination. This notion developed before the significance of the chromosome cycle and meiosis were realized or even known. The antithetic theory, on the other hand, maintained that the sporophyte was an interpolated phase resulting from vegetative growth of the zygote. This theory was described as such well before the correlative event of meiosis was known, yet that phenomenon fit nicely with the idea of interpolation.
The discovery of isomorphic algae further confirmed the genetic homology of the phases that some botanists had long advocated, but it also clouded the issue as to the origin of the sporophyte. It resulted in equating isomorphic phases with the homologous theory, thus making that notion more attractive to its proponents. Since about 1910, however, the homologous theory has generally been assumed to derive the sporophyte by a delay in zygotic meiosis, the same way as the antithetic theory. Botanists adopted the notion of interpolation from the antithetic theory to use for the basic tenet of the homologous theory, taking the main premise of the former to support the latter, without realizing that in doing so they had combined the two theories into one. Curiously, however, many botanists did not recognize the significance of this change in the homologous theory, or even that it had happened. Even supporters of the antithetic theory did not recognize the extent to which the homologous view had co-opted their own. That is probably because by then the emphasis in meaning of the theories had shifted to the morphology of the phases rather than the origin of the sporophyte. The dispute over which theory was right for the origin of alternation continued, however, even though both derived the sporophyte by the same method. For nearly a century the only real difference between the two embattled theories has been whether ancestral sporophytes were isomorphic or heteromorphic. In a sense the old theories have returned to their original meanings, i.e., only suggesting different morphological outcomes of alternation.
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